Radiation therapy, in general, is the use of ionizing radiation for the treatment of disease. The most common use is in the treatment of cancer. The goal of radiation therapy for cancer is to destroy any diseased cells while minimizing the damage to healthy tissue. One device for delivering the radiation to a patent is with a linear accelerator, a machine that generates a high-energy beam of radiation that can be controlled and directed onto specified locations.
A recent development in radiation therapy is intensity-modulated radiotherapy (IMRT) in which the intensity of the radiation delivered is modulated within each field delivered. (Webb, “The Physics of Conformal Radiotherapy”, Institute of Physics Publishing, Bristol (1997)). The purpose of IMRT is to sculpt the radiation dose distribution so that it maximizes the radiation dose to the tumor while maintaining the radiation dose to normal structures within some pre-specified tolerance. (Webb) In IMRT, highly conformal dose distributions can be achieved through the delivery of optimized non-uniform radiation beam intensities from each beam angle. Successful delivery of IMRT can allow for an escalation of the tumor dose and may enhance local tumor control. The dosimetric advantages of IMRT can also be used to provide a reduced probability of normal tissue complications.
Treatment planning for IMRT is typically performed as a two-step process. First, optimized intensity (fluence) maps are determined for each beam direction. An intensity map indicates the pattern of radiation intensity that should be delivered. Next, a leaf-sequencing algorithm is applied that translates the optimized intensity maps into sets of deliverable aperture shapes. Essentially, the leaf-sequencing converts an “ideal” treatment plan into a plan that can be delivered with a treatment unit, such as a linear accelerator.
Because of the complexity of the treatment plan for IMRT, an automated system is required to determine the intensity maps that produce the optimal radiation dose distribution. Currently available IMRT delivery techniques include fixed field beam delivery (IMRT) and intensity modulated arc therapy (IMAT). When radiation is delivered with fixed beam angles, a series of beam shapes are delivered at each beam angle either dynamically, where the leaves of the MLC move during irradiation, or in a step-and-shoot fashion, where the radiation is paused during the movement of MLC leaves. (Convery and Rosenbloom (1992), Bortfeld et al (1994), Yu, Symons, et al (1995); Boyer A. L., and Yu C. X.; (1999);) In contrast, IMAT uses multiple overlapping arcs of radiation in order to produce intensity modulation. (Yu, C. X. (1995); Yu et al (2002)). IMAT can be produced on a conventional linear accelerator (linac) with a conventional multi-leaf collimator (MLC). During each arc, the leaves of the MLC move continuously as the gantry rotates. Moreover, multiple overlapping arcs are used to modulate the intensity of radiation from each beam direction. Rotational delivery provides the distinct advantage of increased flexibility in shaping the dose distribution and allowing better sparing of adjacent critical structures.
Current inverse-planning algorithms for IMRT use a two-step approach (Boyer and Yu 1999). In the first step, the portal that defines the radiation beam's eye view (BEV) for each radiation beam angle is divided into a set number of finite-sized pencil beams. The radiation dose for each of these pencil beams is then calculated and the corresponding beam intensities are subsequently optimized subject to pre-specified treatment goals. The second step uses the radiation intensity maps from each beam angle and translates the radiation intensity maps into a set of deliverable aperture shapes. During optimization of the radiation intensity, the delivery constraints imposed by the design of various components of the linear accelerator are not taken into account, resulting in treatment plans that are often complex and inefficient to delivery. Beamlet-based Inverse Planning (BBIP) irradiates complex target volumes with a large number of small beams. The number of small beams or beamlets is often more than one thousand. The optimization process adjusts the weights of these beams to produce a desired dose distribution.
The two step approach used by current inverse-planning algorithms is unable to generate treatment plans for IMAT. With IMAT, the radiation is delivered while the gantry rotates continuously. Current leaf-sequencing algorithms fail to take the gantry's continuous movement into account. One feature of IMAT treatment plans is that the aperture shapes for adjacent angles within an arc must not significantly differ. This constraint exists because there are limitations on the speed at which the leaves of the multileaf collimator can travel. This constraint makes it difficult to translate the radiation intensity maps into a set of deliverable arcs.
Thus, it would be desirable to provide an arc-sequencing technique that translates optimized intensity maps into deliverable IMAT arcs which can be used to create treatment plans for IMAT.